Could either be vacuum problem or else heater core may be plugged. I had a similar problem of no heat a few years back on my 116 chassis 300SD, ended up replacing the heater core. It was an ugly job, but nothing I haven't seen before. Although the 116 is by far my favorite MB, I really hate that first generation climate control system. That's about the greatest benefit that the Euro spec versions had was the manual adjustment over our ACC system. Ok, hope this helps! I have been through 116 ACC systems many times, so if you need further assistance, please don't hesiate to write.
'67 250SE Cabriolet
'93 300E 2.8